The present invention relates to a process for producing a semiconductor integrated circuit device, and particularly to a technology effectively applicable to the production of a semiconductor memory device wherein the capacitive insulating films of capacitors are formed of a high-dielectric material or a ferroelectric material.
A DRAM (dynamic random access memory) has become the mainstream of large-capacity semiconductor memory devices since it is so simple in memory cell structure as to be easily miniaturized. It is under investigation to use a high-dielectric material of at least 20 in relative dielectric constant, such as Ta.sub.2 O.sub.5 or such as BST {(Ba, Sr)TiO.sub.3 }, or a ferroelectric material in excess of 100 in relative dielectric constant, such as PZT (PbZr.sub.x Ti.sub.1-x O.sub.3), PLT (PbLa.sub.x Ti.sub.1-x O.sub.3), PLZT, PbTiO.sub.3, SrTiO.sub.3, or BaTiO.sub.3 in constituting a DRAM as a countermeasure for making up for a recent decrease in the amount of electric charge storable in a capacitor in keeping with the miniaturization of a memory cell.
On the other hand, in the field of nonvolatile memories, the development of a ferroelectric RAM wherein the polarization inversion of the ferroelectric material mentioned above is utilized for holding memories is in progress. The ferroelectric RAM is expected to be utilizable as a substitute for a flash memory or EEPROM since it permits frequent data rewriting and has a high rewriting speed. Moreover, the ferroelectric RAM is also expected to be utilizable as a substitute for an SRAM for the backup of a battery used in a memory card or the like since it is reduced in electric current consumption and so simple in memory cell structure as to permit high-level integration.
One method of forming a thin film of a high-dielectric or ferroelectric material is a sputtering method wherein an inert gas such as Ar (argon) is impinged against a target made of a hot-pressed sinter of a film-forming material to release clusters of the film-forming material, which are then deposited on a substrate disposed facing the target. In a sputtering method wherein use is made of a target constituted of a high-dielectric or ferroelectric material, particularly a double oxide having a perovskite crystal structure, examples of which include PZT, PLT, and PLZT, however, it is known that the formation of a thin film having a stable composition is difficult for various reasons.
For example, in Japanese Patent Laid-Open No. 249,278/1990, it is pointed out that a thin film of a ferroelectric material having a perovskite crystal structure such as PZT, when formed by sputtering, is liable to the shortage of oxygen in the thin film. A countermeasure to this as disclosed in the above patent gazette is a method wherein a thin film of a ferroelectric material is formed over a substrate by sputtering and then annealed in high-pressure oxygen to effect oxygen uptake into the film to thereby obtain a dense thin film close to the stoichiometric composition and improved in the degree of orientation.
In Japanese Patent Laid-Open No. 272,033/1994 directed to a process for producing a PZT or PLZT target, it is pointed out that an attempt to make the crystal structure of a target homogeneous and fine for the purpose of decreasing the amount of the formed particles causative of short circuit and disconnection of wirings in an LSI complicates the step of hot-pressing a starting material powder to form the target, whereby there arise the problems of contamination of oxygen, etc. Wait impurities and a lot-to-lot variation of oxygen concentration. A countermeasure to this as disclosed in the above patent gazette is the technique of forming a target using a starting material powder of a comparatively large particle size obtained by mechanical alloying.
The above patent gazette also discloses a method wherein the oxygen content of a target is decreased to a value lower than that of the stoichiometric composition to control the oxygen content of the film composition. A target produced by this method is formed into a film by sputtering in an inert gas atmosphere or an oxygen atmosphere, followed by annealing at a temperature of 400 to 700.degree. C. if necessary.
Japanese Patent Laid-Open No. 18,427/1995 and Japanese Patent Laid-Open No. 18,428/1995 both directed to an improvement in a sputtering Pb-containing perovskite crystal target such as PZT, PLT or PLZT disclose the technique of decreasing the localized variation of the Pb content in a thin ferroelectric film, wherein a Pb-containing double oxide and extra PbO which accounts for 5 to 40 wt. % of the whole body are hot-pressed and sintered to produce a target, provided that the extra PbO is mainly constituted of PbO having a tetragonal or rhombic crystal structure.
In "KYOYUDENTAI HAKUMAKU MEMORY (THIN FERROELECTRIC FILM MEMORY)" pp. 187-193, published by Kabushiki Kaisha Science Forum on Jun. 30, 1995, it is pointed out that the formation of a thin PZT-sputtered film involves re-evaporation of Pb due to the temperature or resputtering to present the problem of failure in obtaining a thin film having a stoichiometric composition. A countermeasure to this as introduced by the above-mentioned document is multi-component sputtering equipment wherein a PZT target and a PbO target are simultaneously sputtered to compensate for re-evaporated Pb by PbO, and a method wherein a thin film having a pyrochlore structure is formed by keeping the temperature of a substrate low during sputtering and then annealed to convert that structure,into a perovskite structure.